Filter for photoflash lamps



Jam. 13, 1942. R MUNDER 2,269,984

FILTER FOR PHOTOFLASH LAMPS Filed Oct. 13, 1939 2 Sheets-Sheet 1 RELATIVE ENE BY '6 g E g WAVE LENGTH ILLIMICRIJNS) INVENTUR Husuwam 1f Nl/IYEEZ WAVE LENGTH (MILLI mmnuws) Jan. 113, 1942. R. MUNDER FILTER FOR PHOTOFLASH LAMPS Filed Oct. 13, 1959 2 Sheets-Sheet 2 WAVE LENGTH IN MILLIMIBRDHE 2! 2-0 REEI'PRIJBAL EIF WAVE LENGTH IN MICRUNS IIWE NT 11 1 Jizmwmm 1 Muimm ATTORNEY Patented Jan. 13, 1942 unripe STATES PATENT OFFICE FILTER FOB. PHOTOFLASH LAMPS Rushworth r. Munder, Springfield, Mass. Application October 13, 1939, Serial No. 299,337

4 Claims.

This invention relates to a filter for photoflash lamps, and relates more particularly to a filter adapted for use with a photofiash lamp to produce therefrom light of the spectral characteristic of bright sunlight.

Although it has heretofore been proposed to use filters in connection with photoflash lamps, the previous efforts in this direction have had purposes quite different from those of the present invention and, so far as I am aware, have neither intended nor succeeded in producing light of the character of bright sunlight from a photofiash lamp. For example, Ostermeier, in Reissue Patent No. 18,678 dated December 6, 1932, has described a photofiash lamp provided with a yellow filter, and Kurlander, in Patent No. 2,046,388 issued July 7, 1936, has disclosed providing a photofiash lamp with a blue filterior the purpose of rendering the light emitted less dazzling to the human eye. According to my invention, I provide a photofiash lamp which may be of conventional construction with a filter adapted to change the character of the light transmitted from the normal color temperature of a bare photofiash lamp, which may be in the neighborhood of 4000 K,. to the color temperature of bright sunlight, which may be taken as being 5500 K. The light transmitted by the filter is thus capable of being used for photographic exposures with ordinary daylight film under conditions where such film would not otherwise be usable, and is especially useful for color photography. However, it will be evident to those skilled in the photographic art that such a daylight photofiash lamp may be used wherever it is desired to provide additional light of the character of daylight. For the sake of convenience and simplicity, I preferto utilize my new filter in the form of a coating applied to the inside of the bulb of a photofiash lamp, but it will be evident to those skilled in this art that my filter may be otherwise incorporated in thelamp construction or may take the form of a separate screen to be placed in front of a photofiash lamp.

The principal object of my invention is to 'provide light of the character of daylight from photofiash lamps which are not ordinarily capable of supplying such light.

My invention will be more clearly understood and other objects of my invention will appear from the following description taken inconJunction with the accompanying drawings, in which: Figure 1 is a side elevation, partially broken away, of a photoflash lamp according to this invention.

Figure 2 is a plot of relative energy against wave length in millimicrons having curves each representing light of diil'erent characteristics as described hereinafter.

Figure 3 is a plot of percent of transmittance against wave length in millimicrons having curves each representing a particular filter as described hereinafter.

Figure 4 is a plot oi logarithm of percent transmittance against the reciprocal of wave length in microns having curves each representing a particular filter as described hereinafter.

Referring more particularly to the drawings, the photofiash lamp ill illustrated in Figure 1, has a conventional glass bulb Ii provided with a pressed glass stem i2 and mounted in the usual screw base It. Leads it and i5 extend from the two contact elements in the base through the pressed stem and support a filament it. An igniting compound ii is coated on this filament. The bulb is loosely filled with a combustible materal such as aluminum or aluminum alloy foil, although, as is well known, other materials either in foil, leaf or wire form may be used. As is also well known in this art, the bulb is filled with a suitable combustion promoting gas. The application oi an electric current to the lamp causes the filament to become not, thus igniting the combustible material through the medium of the igniting compound.

The filter to which the present invention is directed is shown in Figure l as a coating on the inside of the bulb, although, as pointed out above, the filter may be a coating on a separate glass plate or other transparent support to be used between the lamp and the point of utilization of the light transmitted by the filter, or may be otherwise incorporated in the lamp structure, as for example, by coating the outside of the bulb.

The curve A of Figure 2 is that of bright noon .sunlight of a color temperature of approximately 5500 K. The color temperature of bright sunlight is of course not an exact quantity, and has been determined as lying in a range of from about 5000 K. to 6500 K. I have chosen 5500" K. as a representative and typical value.

The curve B in Figure 2 is that of light of a color temperature of approximately 4000 K, such as may be emitted by a. clear glass photofiash lamp of the type described without any filter. The color temperature of different photofiash lamps isof course subject to some variation, but 4000 K. is a fair average value as determined by a number of tests on various photofiash lamps.

It will be observed that, as compared to noon sunlight, the light from a clear photoflash lamp is extremely rich in long wave length light, that is to say, light at the red end of the spectrum, and is relatively deficient in short wave length at the violet-blue end of the spectrum. It will be clear from a consideration of curves A and B that in order to provide light from a photofiash lamp approximating that of bright sunlight, we must filter out a substantial part of the longer wave length light. At the same time, the filter should of course be one with a maximum transmission for light at the violet-blue end of the spectrum. Naturally, such filtering of the light results in an appreciable loss of light. In filters constructed in accordance with this invention, such loss is in the neighborhood of 60%, leaving about 40% of the light generated by the lamp transmitted through the filter for utilization. Nevertheless, the advantages of altering the color temperature of the transmitted light are so great, from a photographic standpoint, that this loss is entirely acceptable.

As has been indicated above, the ideal filter would be one having a maximum transmission at the violet-blue end of the spectrum and a minimum transmission at the red end of the spectrum. At first thought, one might expect that a conventional blue dye filter would produce the desired result, but this is not the case. Curve C in Figure 3 is a plot of the percent of transmission against wave length in millimicrons for aniline blue, methyl, which is a typical blue dye, and shows a high transmission at the red end of the spectrum. A filter made from such a dye alone would be wholly unsuitable for the purposes of this invention.

In addition to a maximum transmission at the violet-blue end of the spectrum and a minimum at the red end, I have found that the filter should be one the transmission plot of which descends in a smooth curve so that the percent transmission decreases as the wave length lengthens. The selection and blending of appropriate colors for such a filter is not as simple as might appear at first blush, but I have succeeded in preparing color filters having a very fair approximation of the desired characteristics, as illustrated by curves D, E and F of Figure 3. These curves D, E and F are based upon records made with a recording spectrophotometer of actual light transmission through three separate filters made in accordance with this invention. These three curves and the filters that they represent will be discussed in greater detail hereinafter.

It is customary in the photofiash lamp art to coat the inside of the bulb with a cellulose plastic I lacquer or similar coating to obviate cracking of the glass bulb in service. I find it convenient and entirely practical to combine my filter coating with such a protective coating. A suitable formula for such a combined protective and filter coating is as follows:

lbs. of clear lacquer consisting of butyl acetate having dissolved therein 8 oz. of pyroxylin (nitro-cellulose plastic) per gallon of butyl acetate.

32 lbs. of color lacquer consisting of above clear lacquer with the addition to each gallon of clear lacquer of:

.72 oz. ferri-ferrocyanide blue (Chinese blue) .22 oz. medium chrome yellow .8 lbs. of lacquer solvent consisting substantially of butyl acetate, toluene and ethyl alcohol.

The liquid composition made as above described may be coated upon the inside 0! the bulb before the manufacture of the lamp by filling the bulb with the liquid and then emptying it out and allowing the bulb to drain and dry. With the formula Just given, the viscosity of the liquid is such that a coating of suitable thickness will be formed on the inside of the bulb.

Curve 'D of Figure 3 shows the percent of transmittance against wavelength in millimicrons for a filter consisting of a piece of lamp bulb coated as just described with the foregoing coating formula. The filter represented by this curve D is entirely satisfactory, and photofiash lamps so coated produce light similar in its characteristics to noon sunlight and especially desirable for photographic purposes. Curve E is a similar plot for a filter .made from the same coating composition but applied in an appreciably thicker film on a fiat glass plate. Curve 1' is a similar plot for a filter made from a coating composition containing the same ingredients as the formula given above, but with somewhat different proportions of the two pigments. While these curves-E and F represent usabl filters, the spectral characteristics of the transmitted light are not quite as desirable as for the filter of curve D, as will be shown hereinafter.

In order to provide a standard by which one may appraise the filters of the present invention, it is of interest to consider the characteristics of the ideal filter for the purpose at hand. In the case of a photofiash lamp, we are primarily concerned with the visible spectrum lying between 400 and 700 millimicrons. It will be apparent from a consideration of curves A and B that the intercept of curve B with the 400 miilimicron ordinate, point 9 in Figure 2, determines the maximum energy within this 400400 range that can be obtained from light of 4000" K. when filtered to convert it to light of 5500 K. Starting from this point 9, therefore. we may plot curve G, which is simply a transposition of curve A to this lower energy level. By determining the per cent transmittance at various wave lengths between 400 and 700that would be necessary to reduce the values of curve B to the values of curve G, and plotting the results, we arrive at curve H shown as a broken line in Figure 3. Curve H is thus the transmittance curve of the ideal filter to convert light of the spectral characteristics of curve B to light of the spectral characteristics of curve A.

It will be noted that the transmittance curve D of my preferred actual filter is a good approximation of the ideal curve H, and that this correspondence is particularly close between 460 and 700 millimicrons. at which points curve D has its maximum and minimum.

It has been demonstrated mathematically that the spectral transmission of the theoretical filter to raise light of one color temperature to light of a higher color temperature would be represented by a straight line if one plots the logarithm of the transmittance of the filter against the reciprocal of wave length. It has also been demonstrated mathematically that the slope of this straight line is a measure of the extent to which the filter will raise the color temperature of a light source. For further information regarding said mathematical demonstrations, reference may be made to a published paper by H. P. Gage and Norman Macbeth, entitled "Filters for artificial daylighting, their grading and use," and presented before the thirtieth annual convention of the Illuminating Engineering Society,

Buffalo, New York, August 31 to September 3, 1936.

As pointed out above, Figure 4 is a plot of logarithm of percent transmittance against reciprocal of wave length in microns. The values of wave lengths in millimicrons corresponding to.

the reciprocals are shown at the top of the plot. Curve H of Figure 4 represents the values of curve H of Figure 3 when translated by calculations into the coordinatues of Figure 4. Similarly, curves D, E and F of Figure 4 correspond to curves D, E, and F, respectively of Figure 3. It will be observed that curve H is not exactly a straight line, but approximates the straight line H". The departures of curve H from linearity are no doubt due to errors in the data. from which curves A and B were prepared and to minor inaccuracies in calculation and in constructing the various curves. We may therefore consider the straight line H" as approximately representing the theoretical ideal filter for the purpose in hand.

It will be noted that the slope of the straight line H" is about 30 degrees, this being the angle that the line H" makes with the abscissa. Since the problem solved by the present invention is one of raising the approximately 4000 K. color temperature light of a bare photofiash lamp to light of approximately 5500 K., it will be evident that this slope is an important factor and must be approached with a fair degree of approximation by any actual filter that is successfully to accomplish the desired object. It will beobserved that curve D of Figure 4, which is such a logarithmic plot of the actual filter of curve D, possesses an average slope between the wave lengths of 460 and 700 millimicrons, where curve D has its maximum and minimum, which is a very fair approximation of this slope value of 30 degrees. Curves E and F, which correspond to the actual filters of curves E and F, are likewise reasonably near to the desired average slope value of 30 degrees between the same limits of wave length, it being understood that the departures of all of these curves from linearity must be interpreted by lagarithmic standards, and that the logarithms of small numbers present much larger diflerences than the corresponding numbers themselves.

The essentials in order to produce light of the character of daylight from a photofiash lamp are that the colors of the filter be so selected and combined in such proportions (1) that the resulting filter shall have its maximum transmittance for light at the violet-blue end of the spectrum; (2) that the transmittance of the filter shall follow a smoothly descending curve as the wave length of the light lengthens from said maximum to a minimum at the red end of the spectrum; and (3) that the average slope of the logarithmic :ransmittance curve of the filter between said maximum and said minimum shall approximate .hat of the corresponding theoretical filter. More specifically, it may be said that a filter according to my invention is one having its maxinum transmittance at a wave length shorter .han 500 millimicrons, the transmittance at ;uch maximum being at least 50%, and its minimum transmittance at about 700 millimicrons, vith a smoothly descending curve of transmit- .ance against wave length between such maxinum and minimum, and with the logarithmic ransmittance curve having an average slope rom its maximum to its minimum of about 30 legrees.

It will be noted that the formula specified above utilizes two pigments, namely, an iron blue and medium chrome yellow. Although a suitable iron blue pigment alone would at least approximate the characteristics desired, its transmittance is relatively high in the bluegreen, and th addition of medium chrome yellow cuts down the excess of blue-green transmittance that would be present with the iron blue alone without too greatly increasing the red transmittance. I have also successfully used golden ochre in place of the medium chrome yellow.

I prefer to employ pigments rather than dyes because the pigment colors are substantially fade-proof and will not show any material change in color even though exposed for a period of several months to direct sunlight. However, other pigments and dyes may be used if desired, and I, therefore, do not desire to be limited to the precise pigments herein disclosed as constituting the preferred form of my invention.

As used in the appended claims, the term logarithmic transmittance plot is intended to mean a plot of the logarithm of the percent of transmittance of the filter against the reciprocal of wave length in microns. and the average slope of such logarithmic transmittance plot is intended to mean the average angularity of the line so obtained with respect to the abscissa of such Plot.

Although I have herein described my invention in considerable detail in its preferred form, I desire to be limited only by the prior art and the scope of the appended claims.

I claim:

1. In a photofiash lamp particularly for photographic purposes and comprising a glass envelope and a fiash light producing charge located therein, in combination, a color filter comprising a color layer including a blue color material and a yellow color material. coated on said glass envelope, said filter having its maximum transmittance for light of a wave length shorter than 500 millimicrons and a transmittance for longer wave length light decreasing in such manner that the plot of the transmittance of the filter against the wave length of light forms a smooth curve, the transmitted energy of said filter at its maximum transmittance being at least 50 percent, and the logarithmic transmittance plot of said filter having an average slope of about 30 degrees between the reciprocal of the wave length at which said filter has its maximum transmittance and the reciprocal of the wave length of 700 millimicrons, whereby to produce from said photofiash lamp light of a character similar to that of bright sunlight.

2. In a photofiash lamp particularly for photographic purposes and comprising a glass envelope and a flash light producing charge located therein, in combination, a color filter comprising a color layer including an iron blue pigment and a yellow pigment coated on said glass envelope, said filter having its maximum transmittance for light of a wave length shorter than 500 millimicrons and a transmittance for longer wave length light decreasing in such manner that the plot of the transmittance of the filter against the wave length of light forms a smooth curve, the

transmitted energy of said filter at its maximum transmittance being at least 50 percent, and the logarithmic transmittance plot of said filter having an average slope of about 30 degrees between the reciprocal of the wave length at which said light of a character that of-brightsunlight.

filter has its maximum transmittance and the from said photofiash' lamp similar to that of bright whereby to produce sunlight.

3. The combination with a phctofiash lamp particularly for photographic purposes and com prising a glass envelope and a flash light produc- I reciprocal of the wave length of. 700 millimicrons, I I

graphic purposes and comprising a glass envelope, and a flashlight producing charge located therein,- in combination, a color filter comprising a color layer including coated on said glass envelope, said filter having its maximum transmittance for light of a'wave .-length shorter than 500 millimicrons and a ing charge located therein, of a combined profilter coating on the inside 02 comprising a cellutective and color I said envelope, said coating lose lacquer, pigment, and said coating having its maximum transmittance-for light of a wave length shorter than 500 millimicrons and a transmittance for longer wave length light 'decreasingin such man- 4. In a photoflash lamp pa ticularly for photoan iron blue pigment and a yellow.

transmittance tor longer'wave length light decreasing in'such manner transmittance of the fllteragainst the wave length of light forms a smooth curve, the trans- .mitted energy 0! said filter at its maximum transmittance being at least 50 percent, and the logarithmic transmittance plot of said filter havins-an average slope of about 30 degrees between the reciprocal of the wavelengthjat which said filter has its maximum transmittance and the reciprocal of the wave length of 700 miliimicrons,

whereby to produce from said photoflash lamp light of a character similar to thatot bright suna light.

-RUSHWORTH I. MUNDER.

a blue color I material that the plot of the 

